Aims: The purpose of this study was to evaluate and compare the sealing ability and the thermal insulating capability of four different cavity lining materials. Materials and Methods: Forty noncarious human mandibular second premolars that were extracted for orthodontic treatment were collected, cleaned, and stored in distilled water. These premolars were randomly divided into four groups of ten teeth each for treatment with the different cavity lining materials. Group I teeth were treated with cavity varnish, group II teeth with amorphous calcium phosphate (ACP), group III teeth with dentin bonding agent, and group IV teeth with resin-modified glass ionomer cement (RMGIC). Electrical resistance and the difference in the time-temperature curve of the external surface and the pulp side [A D -A P ] of each tooth following heat and cold application for 120 s were measured before and after cavity lining placement to determine the sealing ability and thermal insulating property, respectively. Data collected were subjected to statistical analysis. For paired data, paired t-test and Wilcoxon's signed rank test were used. One-way ANOVA was used for comparisons between multiple groups and the Mann-Whitney U test for comparisons between pairs. Results: The mean difference in electrical resistance (in KΩ) of different cavity lining materials were as follows: group I = +3.53, group II = −1.00, group III = +20.43, and group IV = +11.44. The mean differences in the area (A D -A P ) under the time-temperature curve following heat application were as follows: group I = 6.6 mm 2 , group II = 15.3 mm 2 , group III = 130.5 mm 2 , and group IV = 412.0 mm 2 . The mean differences in the area (A D -A P ) under the time-temperature curve following cold application were as follows: group I = 24.5 mm 2 , group II = 3.2 mm 2 , group III = 314.9 mm 2 , and group IV = 480.5 mm 2 . Conclusion: Dentin bonding agent and RMGIC provided effective sealing of the dentinal tubules and significant thermal insulation when compared to the other tested cavity lining materials.

Preservation of vitality of the tooth is an essential objective of any operative procedure. Enamel and cementum provide protective covering over dentin. Cavity and crown preparation are likely to cause loss of these protective barriers, resulting in exposure of dentin and the severing of its tubules. Once the tubules are opened they act as channels that transmit mechanical, chemical, and bacterial stimuli to the pulp and are also susceptible to hydrodynamic effects.[1]

Materials used to restore the teeth are expected to seal these tubules. However, most restorations have a gap between the wall of the preparation and the restorative material; this can permit oral bacteria to colonize the gaps and to shed bacterial products onto the pulp, thereby causing pulpal damage.[2]

Another cause of pulpal damage is repeated chronic exposure to thermal changes. It was clinically recognized that recently filled teeth frequently were more thermosensitive than their neighbors. Although operative trauma undoubtedly plays a role in this, it was thought that more rapid conduction of heat by the metallic restorations could only compound the problem, especially when the thickness of the remaining dentin is reduced.[3]

Hence, cavity lining materials, which are used as pulp protective measures, should possess two important properties: it should have low thermal diffusivity and it must provide effective dentinal tubule sealing. Currently available pulp protecting materials can be divided into sealers, liners, and bases. The clinician must decide prior to placing or cementing a restoration into a cavity preparation, if a sealer, liner, or base should be placed on the cavity walls. While apparently simple, this decision has been complicated by an ever-increasing number of products made available.

Cavity varnish has been utilized for many years to seal the restoration-tooth interface until corrosion products are formed to eliminate the gap. The materials used as cavity sealers these days are those that have demonstrated multisubstrate bonding ability to bond the restorative material to the tooth. These include glass ionomers and bonding agents. Bonding agents are capable of coating the dentin with a hybridized layer and thereby can reduce dentin permeability. Although research on the use of dentin bonding agents has shown promising results, the data are often equivocal and rarely definitive. Glass ionomer cements have been utilized as cavity liners in an attempt to take advantage of their ability to form a chemical bond and to release fluoride.[4] Amorphous calcium phosphate (ACP) has been recently introduced as a cavity lining material. It seals dentinal tubules by occluding them by precipitation of amorphous calcium phosphate, thereby reducing dentinal sensitivity.[5]

Hence, the objectives of the present study were to evaluate and compare the sealing ability and the thermal insulating capability of four different cavity lining materials, viz., cavity varnish, ACP, dentin bonding agent, and resin-modified glass ionomer cement (RMGIC). The null hypothesis tested was that there is no difference between these different cavity lining materials with regard to their sealing and thermal insulating properties.

Materials and Methods

Forty noncarious human mandibular second premolars that had been extracted for orthodontic treatment were collected, cleaned, and stored in distilled water. These premolars were randomly divided into four groups of ten teeth each. Each group was treated with a different cavity lining material:

A standardized occlusal cavity of 3.0 × 3.0 × 4 mm was prepared. The greatest depth, width, and length of the newly formed cavities were measured using a William's graduated periodontal probe. The root was cut off 1 mm above the apex and the root canal was enlarged using endodontic K-files, employing a retrograde approach to facilitate insertion of an electrode or a thermocouple. Following cavity preparation, all the teeth in the four groups were subjected to electrical resistance and temperature change measurement.[6]

Electrical resistance measurement

The pulp chamber of each tooth was filled with saline solution and a copper electrode was inserted into the pulp chamber from the apex. The crown was then submerged in saline solution. A sinusoidal voltage (50 Hz, 5 Vp-p) was applied to the experimental circuit from a function generator [Figure 1].

The voltage drop caused by the standard resistor (1000 Ω) in the circuit was measured by a millivoltmeter. The resistance (KΩ) between the electrode placed inside the pulp chamber and that in saline, i.e., the resistance of the tooth, was calculated using the formula,

R t = Vt/I

Where R t = resistance of the tooth, V t = voltage drop across the tooth, and I = the current in the circuit. The values of voltage drop caused by the tooth (V t ) and the current in circuit (I) were obtained using the following formulae:

Temperature change measurement

Chromium-aluminum thermocouples of 0.9-mm diameter connected to digital output (Frontier Process Control, Bangalore) were used for this experiment. One thermocouple was secured on the pulpal floor of the occlusal cavity (TC-1) and another was inserted from the apex and brought into contact with the roof of the pulp chamber (TC-2). The pulp chamber was filled with distilled water and sealed with sticky wax, and plaster of paris was applied over that for better sealing. The root of the tooth was immersed in water at a temperature of 37°C. Cold / hot water was run into the coronal part; thus, water from either an ice bath (4°C) or hot water bath (60°C) was applied to the coronal surface of the tooth for at least 120 s to introduce the thermal stimulus. The change in temperature due to the thermal stimulus was recorded with a digital output indicator every 5 s [Figure 2].

The data were fed into an analytical programmer (AutoLIS P in AutoCAD 2005). The time-temperature curves were plotted and are shown in Graphs 1-4 (These sample graphs represent a specimen from group IV, i.e., the group that received RMGIC). The area under the time-temperature curve for the pulp side (A P ) was subtracted from that for the tooth surface side (A D ) and the absolute value (A D -A P ) was determined to express the magnitude of the thermal diffusion through the dentin.

Onto the prepared cavities one of the following cavity lining materials were applied:

Group I: Cavity varnish

Using a cotton pellet and tweezers, two coats of varnish were applied onto all the cavity walls and to the pulpal floor.

Group II: Amorphous calcium phosphate

Amorphous calcium phosphate is available in two parts: part A and part B solutions. A few drops of part A solution were dispensed into well A. The disposable applicator was soaked with part A solution and rubbed gently on the pulpal floor for 5-10 s. Following this, a few drops of part B solution were dispensed into well B. Another new applicator was soaked with this solution and rubbed gently for 5-10 s on the pulpal floor treated with part A solution.

Group III: Dentin bonding agent

Appropriate amounts of primer and bond were dispensed. Generous amount of primer was applied on to the cavity walls and left for 20 s. Mild air blow was used to dry the primer; following this, bond was applied, distributed evenly in the preparation using mild air blow, and light cured for 10 s.

Group IV: Resin-modified glass ionomer cement

The powder and liquid were mixed according to the manufacturer's instructions and applied onto the dentin wall in a thin layer of 0.5 mm and then light cured. The thickness of the RMGIC liner was confirmed using a customized William's graduated probe.

Following cavity lining, all the four groups were again subjected to electrical resistance and temperature change measurement as explained earlier. Electrical resistance values and temperature change measured prior to lining the cavities were used as control against which the values obtained following cavity lining application were compared.

Data collected were subjected to statistical analysis. For paired data, paired t-test and Wilcoxon's signed rank test were used. One way ANOVA was used for comparisons b/w multiple groups and the Mann-Whitney U test for comparisons between pairs.

Results

The means, standard deviations, and P values for change in electrical resistance and difference in the area [A D -A P ] under the time-temperature curve following heat and cold application are presented in [Table 1],[Table 2],[Table 3], respectively. The results showed that the maximum increase in electrical resistance was achieved with dentin bonding agent, followed by RMGIC and cavity varnish. Increase in the electrical resistance following application of these cavity lining materials were statistically significant ( P < 0.01). However, in contrast to these materials, AC P showed statistically significant reduction in electrical resistance ( P < 0.01).

RMGIC showed 38.9% (< 0.01) increase in A D -A P following heat application and 20.3% (< 0.01) increase following cold application. This increase was more than that with any of the other cavity lining materials and was statistically significant, suggesting effective thermal protection for the pulp. Dentin bonding agent also showed increase in A D -A P , which was 10.5% (< 0.05) and 12.8% (< 0.01) following heat and cold application, respectively. Cavity varnish and ACP did not reduce the magnitude of temperature change significantly. There was no statistically significant difference in the area under the time-temperature curve (A D -A P ) before cavity lining, either with heat application ( P = 0.33) or with cold application ( P = 0.38).

Discussion

Conductivity across dentin is assumed to be the result of transportation of ions across dentinal tubules.[7] The increase in electrical resistance of the tooth specimen suggests less ion transport between dentin and physiologic saline solution. This in turn would indicate effectiveness of the material in blocking the passage of ions, bacteria, chemical substances, and environmental fluids.[6]

The present study revealed that the use of dentin bonding agent increased the electrical resistance 5.16 fold. It also revealed that the maximum increase in the electrical resistance was caused by dentin bonding agent when compared to the other materials tested. The difference was statistically significant. This can be explained on the basis of the ability of dentin bonding agents to coat the dentin by forming a hybrid layer by the morphologic impregnation of demineralized vital dentin with a hydrophilic resin monomer.[6],[8] The adhesive system tested in the present study is well known to hybridize with dentin. The proprietary primer has shown excellent hydrophilic property and the proprietary bonding resin is hydrophobic; this enables the formation of a durable hybrid layer.[9]

RMGIC caused a 3.8-fold increase in the electrical resistance, which was statistically highly significant. However, this increase was lower than that produced by the dentin bonding agent. This was expected, because even after setting, glass ionomer cements remain highly ionic and conductive.[10]

Though the increase in electrical resistance after cavity varnish application was highly significant, it was lower than that produced by the dentin bonding agent and RMGIC. This can be explained on the basis of discontinuities in the applied varnish layer and the hydrophobic nature of the cavity varnish resulting in poor dentin wetting capabilities.[11]

Only ACP showed reduction in electrical resistance following its application. This may be because of its ionic nature. However, the result obtained with the use of ACP is in contrast to a previous study done by Tung et al.[5] in which there was marked decrease in dentin permeability, as measured using hydraulic conductance, after use of calcium phosphate. This can be attributed to the difference in the technique employed to measure the sealing ability. Moreover, an in vivo split-mouth study done by Yates et al. (1998) showed no significant therapeutic difference between placebo and ACP on dentinal hypersensitivity.[12] Decrease in sensitivity observed in vivo can be attributed to the formation of fibrin seals between damaged odontoblasts and dentinal tubules, which reduce the ability of exogenous molecules to reach the pulp.[13] Moreover, in vivo studies rely on subjective symptoms to evaluate the therapeutic effect of desensitizing agents.[12] It is not surprising that in vivo clinical experience may not correlate with in vitro studies as some of the biologic reactions do not occur in vitro .[13] These explanations, while possible, must remain speculative until tested by further scientific research.

In the present study, to simulate consumption of hot or cold beverages, temperatures of 60°C and 4°C were employed for a period of 120 s to allow the intrapulpal temperature to attain a steady state, while the tooth was immersed in water which was maintained approximately at 37°C to simulate the intraoral condition. However, it was not possible with our experimental setup to maintain the temperatures of the thermal baths at a constant level for long periods of time and hence there was 1-2°C variation in temperature.

The difference between the areas under the time-temperature curve of the surface and that of the pulp (A D -A P ) was assumed to express the degree of thermal diffusion through the dentin.[6],[14] This in turn was assumed to reflect the thermal insulating effect. Therefore significant increase in A D -A P would suggest that the cavity lining material provided effective thermal protection for the pulp.[6]

RMGIC showed the maximum increase in A D -A P following heat application and cold application when compared to the other cavity lining materials, suggesting effective thermal protection for the pulp. Dentin bonding agent also showed increase in A D -A P following heat and cold application. However, this increase was lower than that produced by RMGIC. This is because the thermal insulating efficacy is to some extent directly related to the cross-sectional thickness of the material. These thermal insulating effects are clinically important in minimizing the discomfort that the patient may experience during consumption of hot / cold food or drink especially under metallic restorations.

Cavity varnish and ACP did not show any significant increase in A D -A P . This is because they were very thin in cross-section.[15],[16] The slight increase in mean value was statistically not significant.

Conclusion

Thus, dentin bonding agent and RMGIC can be expected to seal dentinal tubules effectively and provide significant thermal protection. Therefore, they are expected to perform as good cavity lining materials.

However, the efficacy of these materials should be evaluated further with in vivo studies before drawing definitive conclusions. Moreover, ACP which has been reported as providing promising results needs to be further evaluated by alternative methods.

Acknowledgment

We would like to thank Mr. Pradeep Kumar Dixit, Senior Lecturer, BIET, Davangere, for helping us through the experiments.